The present invention comprises an apparatus and method for performing surgery on the cornea by electronically measuring the thickness of the cornea and automatically or manually adjusting the depth of a surgical blade in accordance with the measured thickness. Surgical treatment of the eye in numerous instances requires a partial thickness incision of the cornea. Examples where such incisions are required, include radial keratotomy, relaxing incisions, wedge resections, lamellar keratoplasty, tumor excisions, etc. The apparatus disclosed herein improves the safety and accuracy with which these procedures can be executed. Although the apparatus will have value in the performance of other procedures, its initial application will be in the area of radial keratotomy.
Radial keratotomies involve the reshaping of the cornea to improve refractive error. In 1974, Fyodorov developed a technique in Moscow for making partial thickness radial corneal incisions for the reduction of myopia (nearsightedness). Since that time, this procedure has been performed on approximately 1500 persons in the Soviet Union. A review of the risks and benefits of this procedure is expected to be published in the Annals of Opthamology. It is anticipated that a major problem inherent in the procedure will be the possibility of corneal perforation. If such perforation occurs, there is a risk of infection and ensuing visual loss. One purpose of the present invention is therefore to reduce greatly this risk.
Another problem in the performance of corneal surgery involves the regulation of depth of the incisions which must be precise in order to correct refractive error accurately. The apparatus disclosed herein helps in this regard.
In the prior art, for example, U.S. Pat. No. 4,154,114, ultrasonic pulses have been employed to determine distances between points in the body. However, most instruments previously used to measure distances within the eye utilize an ultrasonic probe having a tip which touches the cornea in a manner which can introduce small but significant errors. These instruments use the probe tip echo to represent the anterior cornea surface. But in fact, the probe tip is seldom totally coincident with the surface of the cornea, and so the opportunity for an introduced error is possible in the measurement.
Another problem in the use of ultrasonic techniques to measure corneal thickness is the fact that the cornea is extremely thin. Pulses reflected from the anterior cornea surface (i.e. the outer surface) and the endothelium surface (i.e. the inner surface) are separated by such a short interval as to make it difficult to accurately measure the time interval between pulses using classical techniques. Although most ocular element distances are of the order of 2-25 mm, the human cornea is only about 0.4-1.0 mm thick, so the time between echo pulses can be as short as 500 nanoseconds. In order to measure the distance between pulses, a clock frequency of approximately 80 MHz would be required instead of the typical 8 MHz clock frequency used in present ocular element measurement systems.
The requirement of a high frequency clock introduces several technical problems. Standard transistor-transistor logic (TTL) elements can no longer be used in gating and counting circuits because the maximum operating frequency for TTL is 40 MHz. Secondly, emitter-coupled logic (ECL) could be used, but ECL is very susceptible to noise transients and the complexity of the circuit board layout is greatly increased.
One approach that has been suggested for solving the high clock frequency problem discussed above would be to average individual measurement cycles at a lower clock rate. With this method, instead of the counter being reset after each single real-time pulse measurement, the counter is allowed to count up for a predetermined number of pulse echo cycles (such as 100) and therefore, the clock frequency could be decreased. This approach presents at least three problems. First, real-time measurements are not possible. Secondly, the accuracy of the measurement is a function of the number of samples taken. Greater accuracy can sometimes be obtained with a larger sample, but at the expense of more time. In certain approaches to corneal surgery, this extra time may not be available, as it is desired to adjust the cutting blade almost instantaneously based on the local measured corneal thickness. Thirdly, to use a statistical averaging approach, it is mandatory that the clock frequency be totally independent of the circuit which generates the ultrasonic pulse signals. Due to the radio frequency (rf) energy which is emitted while generating these pulses, it is extremely difficult to prevent this energy from influencing the counter frequency, thereby degrading the statistical average.